To achieve higher Speeds, just add the addvalue 2 or 3
or more times per frame.

I can see this eating up a little too much time during VBLANK to iterate
over the "add" loop several times to accomplish really fast speeds, but
I'll start experimenting with 0-1 pixels per frame and see if I can get the
car moving in the 16 proper motion vectors at a constant speed, then go
from there.